Huge Sumatran quakes in April a step in tectonic plate breakup

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LOS ANGELES—Planet Earth may be 4.5 billion years old, but that doesn’t mean she can’t serve up a shattering surprise now and again.

Such was the case on April 11 of this year when two massive earthquakes erupted beneath the Indian Ocean, far from the usual danger zones.

Now scientists say the seafloor ruptures are part of a long suspected, yet never before observed, event: the slow-motion splitting of a vast tectonic plate.

The first of the quakes, a magnitude 8.7, was 20 times more powerful than California’s long anticipated “big one” and tore a complex network of faults deep in the ocean floor.

The violence also triggered unusually large aftershocks thousands of miles away, including four off North America’s western coast.

“It was jaw-dropping,” said Thorn Lay, a professor of Earth and planetary sciences at the University of California, Santa Cruz. “It was like nothing we’d ever seen.”

At first, Lay wondered whether the computer code he used to analyze earthquakes was wrong. Eventually, he and other scientists realized that they had documented the break-up of the Indo-Australian plate into two pieces, an epic process that began roughly 50 million years ago and will continue for tens of millions more. Lay and other scientists reported their findings online Wednesday in the journal Nature.

Most great earthquakes occur along plate borders, where one plate dives beneath the adjoining plate and sinks deep into Earth’s mantle, a process called subduction. The April 11 quakes, however, occurred in the middle of the plate and involved a number strike-slip faults, meaning the ground on one side of the fault moves horizontally past ground on the other side.

Scientists say the 8.7 main shock broke four faults. The quake lasted 2 minutes and 40 seconds — most last just seconds — and was followed by a second 8.2 main shock two hours later.

Unlike the magnitude 9.1 temblor that struck in the same region on Dec. 26, 2004, and created a deadly tsunami, the April 11 quake did not cause similar destruction. That’s because horizontally moving strike-slip faults do not induce the massive, vertical displacement of water that thrust faults do on the borders of plates.

The type of interplate faults involved in the Sumatran quakes are the result of monumental forces, some of which drove the continent of India into Asia millions of years ago and lifted the Himalayan Mountains. As the Indo-Australian plate continues to slide northwest, the western portion of the plate, where India is, has been grinding against and underneath Asia. But the eastern portion of the plate, which contains Australia, keeps on moving without the same obstruction. That difference creates squeezing pressure in the area where the quakes occurred.

The study authors say that over time, as more quakes occur and new ruptures appear, the cracks will eventually coalesce into a single fissure.

“This is part of the messy business of breaking up a plate,” said University of Utah seismologist Keith Koper, senior author of one of the studies. “Most likely it will take thousands of similar large quakes for that to happen.”

The quake was also notable for triggering powerful aftershocks thousands of miles away. While major quakes have been known to trigger aftershocks at great distance, they are usually less than 5.5 in magnitude. The April earthquake triggered 11 aftershocks that measured 5.5 or greater in the six days that followed the main shock, including one as big as magnitude 7. Remote shocks were felt 6,000 to 12,000 miles from the main quake.

Fred Pollitz, a geophysicist with the U.S. Geological Survey in Menlo Park, Calif., and lead author of one of the studies, said the quake was extremely effective in transmitting seismic wave radiation around the world. Though the magnitude of the Sumatran quake is No. 10 on the list of historic quakes, Pollitz said no other quake has triggered so many strong aftershocks so far away.

“It’s the most powerful earthquake ever in terms of capability of putting stress on other fault zones around the world,” he said.

Pollitz said the quake is likely to teach seismologists about the physics of earthquakes, particularly those along strike-slip faults. That knowledge, he said, would certainly apply to the San Andreas fault, which is also a strike-slip fault.

Lay said that the quake was most surprising in that it was completely unanticipated by seismologists and that he did not expect to event to repeat itself any time soon.

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